Plasmodium merozoite actively enter host erythrocytes by a complex but poorly understood invasion process. Molecules from the micronemes, an organelle of the merozoite apical complex, appear to facilitate parasite invasion by binding to specific receptors on the erythrocyte's surface. Different Plasmodium species use different erythrocyte receptors, but these parasite molecules which recognize the different receptors are genetically related. Members of this family of erythrocyte binding proteins, herein termed merozoite Microneme Protein-1 (MP-1), include the P. vivax and P. knowlesi Duffy binding protein family and the P. falciparum sialic acid binding protein (EBA175). These proteins have intrinsic interest for their biological function and also because antibody to the EBP175 has been shown to inhibit parasite development. Identification of conserved and immunogenic regions of these proteins which induce an inhibitory immune response will be important for the development of an asexual stage malaria vaccine. Genetic analysis of the MP-1 gene family is necessary to identify conserved and polymorphic regions. Towards this goal I have examined laboratory strains of P. knowlesi and field isolates of P. vivax. I have identified polymorphisms in the MP-1 genes of both species. I propose to further characterize polymorphism in field isolates of P. vivax in order to determine if the vivax MP-1 has more than one allele and to identify conserved and polymorphic regions of the MP-1. Laboratory models for the study of the MP-1 family will be established using the rodent malaria parasites P. berghei and P. yoelii. The MP-1 genes of P. berghei and P. yoelii have been identified in my laboratory and will be cloned and characterized. The MP-1 genes of P. vivax, P. berghei and P. voelii will be expressed as recombinant proteins and used to assay immune reactions to MP-1 in their respective hosts. Full length and separate regions of the MP-1 will be expressed as recombinant proteins and tested for growth inhibition in vaccinated animals. Monoclonal antibodies will be produced against the P. berhei MP-1 for use in characterizing immunogenic epitopes and functional regions. The experiments outlined in this proposal will help identify and characterize the essential functional regions of these important erythrocyte binding proteins so they can be developed as effective vaccines.